Polyolefins such as high density polyethylene (HDPE) homopolymer and linear low density polyethylene (LLDPE) copolymer can be produced using various combinations of catalyst systems and polymerization processes. Metallocene-based catalyst systems can, for example, produce ethylene polymers having good impact strength, tear resistance, and optical properties, but often at the expense of poor extrusion processability and melt strength. Chromium-based catalyst systems can, for example, produce ethylene polymers having good extrusion processability and polymer melt strength in certain pipe applications, typically due to their broad molecular weight distribution (MWD).
However, it can be difficult for metallocene-based and chromium-based catalyst systems to produce polymers that can be successfully extruded into large diameter and thick wall pipe products. Accordingly, it is to this end that the present invention is generally directed.